1. Field of the Invention
The present invention relates to a die cushion device of a press machine which drives up and down a cushion pad by a servomotor.
2. Description of the Related Art
The press machine is provided with a die cushion device (hereinafter simply called as the die cushion) which is used to suppress wrinkles during drawing. A conventional die cushion produces a cushion pressure while driving up and down a cushion pad by using a hydraulic pressure or an air pressure. To enhance drawability of the press machine and to prevent a work from being broken or distorted, it is necessary to control the cushion pressure of the die cushion with high accuracy, especially it is necessary to control the cushion pressure with high accuracy when the cushion pad is moved downward.
The die cushion using an air pressure only cannot control the cushion pressure very accurately when the cushion pad operates. The die cushion using a hydraulic pressure can control the cushion pressure very accurately by controlling a pressure oil when the cushion pad operates. But, the hydraulic equipment has drawbacks that its structure is complex, and precise maintenance and control are required. Therefore, the die cushion provided with an electric servomotor having a simple structure and not requiring precise maintenance or control is receiving attention in these years.
Japanese Patent Application Laid-Open No. 6-544 (hereinafter called as “patent literature 1”) discloses a die cushion which is provided with a rotary electric servomotor. This die cushion is generally comprised of a cushion pad and a drive mechanism which drives the cushion pad. The drive mechanism is generally comprised of a servomotor and a power transmission mechanism which transmits the power of the servomotor to the cushion pad. The power transmission mechanism is generally comprised of a support rod, a rack and a pinion.
A support rod is connected to the bottom surface of the cushion pad, and the rack is connected to the lower part of the support rod. The cushion pad, the support rod and the rack are integrally movable up and down. The pinion is occluded to the rack, and the pinion is coupled to the rotation shaft of the servomotor. The servomotor is supplied with an electric current to rotate the rotation shaft, the pinion is rotated, and the rotation of the pinion causes the rack to move up and down. The support rod and the cushion pad also move up and down together with the rack.
Japanese Patent Application Laid-Open No. 6-543 (hereinafter called as “patent literature 2”) discloses a die cushion which is provided with a rotary electric servomotor in the same manner as in the patent literature 1. The cushion pad described in the patent literature 2 is divided into plural portions, and the individual divided cushion pads are coupled to the servomotor via a rack and pinion mechanism and a train of reduction gears. And, the individual servomotors are controlled to move up and down the cushion pads.
The size of the cushion pad and the ability of the die cushion are determined according to the request of the user. Therefore, the die cushion is designed according to the specifications as required. And, it is natural to change a design of a drive mechanism depending on a limitation of *a mounting space* of the cushion pad and the press machine, and there are many occasions that the engineering change to the bed frame is forced. Therefore, the design man-hours increase in the die cushion production stage.
For example, the die cushions disclosed in the patent literature 1 and the patent literature 2 have a drive mechanism which is large in structure in the vertical and horizontal directions. If this drive mechanism cannot be housed, it becomes necessary to make engineering changes or the like of the drive mechanism in the *mounting space* allowed by the user. Then, it becomes necessary to have the above-described number of design man-hours.
The die cushion disclosed in the patent literature 2 has each of the plural divided cushion pads independently controlled by the servomotors. Because the cushion pads are divided, the cushion pressure can be changed partly, and it is advantageous. But, the dividing parts of the cushion pad are determined according to a request by the user, so that they are different among the individual die cushions. In other words, it is necessary to design the die cushion according to the specifications as required. In this connection, it can be said that the number of man-hours to design the die cushion increases as described above.
Besides, a high capacity die cushion requires a large drive mechanism. As a result, the component elements of the drive mechanism increase. Then, types of used parts increase, then it becomes necessary to manage the various parts, and the management cost increases.
As described above, the production of a conventional die cushion may have problems that the number of man-hours to design increases and the cost becomes high.
The present invention has been made in view of the above circumstances and provides a die cushion device which is inhibited the increase of the number of types of used parts by facilitating the design of the die cushion and miniaturizing the drive mechanism of the die cushion.